This invention relates generally to fluid processing, and specifically to process flow measurement and control. In particular, the invention concerns magnetic flowmeters.
Magnetic flowmeters (or mag meters) measure flow by Faraday induction, an electromagnetic effect. The meter energizes a coil (or coils) to generate a magnetic field across a pipe section, and the magnetic field induces an electromotive force (EMF) across the process flow. The flow velocity is proportional to the induced EMF, and the volumetric flow rate is proportional to the flow velocity and flow area.
In general, electromagnetic flow measurement techniques are applicable to water-based fluids, ionic solutions and other conducting liquid flows. Specific uses include water treatment facilities, high-purity pharmaceutical manufacturing, hygienic food and beverage production, and chemical processing, including hazardous and corrosive process flows. Magnetic flowmeters are also employed in the hydrocarbon fuel industry, including hydraulic fracturing techniques utilizing abrasive and corrosive slurries, and in other hydrocarbon extraction and processing methods.
Magnetic flowmeters provide fast, accurate flow measurements in applications where differential pressure-based techniques are disfavored because of the associated pressure drop (for example, across an orifice plate or Venturi tube). Magnetic flowmeters can also be used when it is difficult or impractical to introduce into the process flow a mechanical element, such as turbine rotor, vortex-shedding element or Pitot tube.
A magnetic flowmeter determines a flow rate of a conductive fluid flowing through a conduit or pipe by measuring a voltage generated across the fluid in a direction perpendicular to the fluid flow as the fluid moves through a magnetic field generated by the flowmeter. The voltage is measured between two electrodes that are in contact with the fluid and are positioned on opposite sides of the pipe. The pipe walls must be either electrically non-conductive or, if conductive, have an electrically non-conductive liner to keep from shorting out the voltage generated across the fluid flow. If the pipe wall is conductive, the two electrodes must also be electrically insulated from the pipe wall and must penetrate the non-conductive liner to accurately measure the generated voltage.
The magnetic flowmeter requires electrodes to carry voltage from the process fluid to a transmitter. Some of the key attributes desired by customers with respect to the electrodes are low profile (low noise), low cost, compatibility of materials, coating resistant (non-sticking), and a wide pressure and temperature performance range.
The application of magnetic flowmeters in the oil and gas industry present challenges because the magnetic flowmeter linings and electrodes can be exposed to high pressures. This is particularly the case with polytetrafluoroethylene (PTFE) linings because of the tendency of PTFE to “cold flow”, whereby the PTFE lining expands and contracts under pressure and temperature. The result can be that the PTFE liner and the electrodes can separate and process fluid can find leak paths that compromise the electrodes.